Incidence of the introduced parasitoids Cotesia kazak and Microplitis croceipes (Hymenoptera: Braconidae) from Helicoverpa armigera (Lepidoptera: Noctuidae) in tomatoes, sweet corn, and lucerne in New Zealand

Morphological defense traits of plants such as trichomes potentially compromise biological control in agroecosystems because they may hinder predation by natural enemies. To investigate whether plant trichomes hinder red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae), as biological control agents in soybean, field and greenhouse experiments were conducted in which we manipulated fire ant density in plots of three soybean isolines varying in trichome density. Resulting treatment effects on the abundance of herbivores, other natural enemies, plant herbivory, and yield were assessed. Trichomes did not inhibit fire ants from foraging on plants in the field or in the greenhouse, and fire ant predation of herbivores in the field was actually greater on pubescent plants relative to glabrous plants. Consequently, fire ants more strongly reduced plant damage by herbivores on pubescent plants. This effect, however, did not translate into greater yield from pubescent plants at high fire ant densities. Intraguild predation by fire ants, in contrast, was weak, inconsistent, and did not vary with trichome density. Rather than hindering fire ant predation, therefore, soybean trichomes instead increased fire ant predation of herbivores resulting in enhanced tritrophic effects of fire ants on pubescent plants. This effect was likely the result of a functional response by fire ants to the greater abundance of caterpillar prey on pubescent plants. Given the ubiquity of lepidopteran herbivores and the functional response to prey shown by many generalist arthropod predators, a positive indirect effect of trichomes on predation by natural enemies might be more far more common than is currently appreciated.     

COST OF GLANDULAR TRICHOMES,A “RESISTANCE” CHARACTER IN DATURA WRIGHTII REGEL (SOLANACEAE)

Models regarding the evolution of plant resistance to herbivory often assume that the primary mechanism maintaining resistance polymorphisms is the balance between benefits of increased resistance to herbivores and costs associated with the production of a resistance character. However, rarely has it been demonstrated that genetically based resistance traits are costly. Here, we document costs associated with the production of glandular trichomes, a resistance character in Datura wrightii that is predominantly under the control of a single gene of large effect. In the absence of herbivores, plants with glandular trichomes (sticky) produced 45% fewer viable seeds than plants with nonglandular trichomes (velvety). Although both plant types flowered with similar frequency, sticky plants matured fewer capsules and fewer of their seeds germinated. The fitness difference between the types in herbivore-free conditions was not mitigated by the addition of water, a potentially limiting resource for sticky plants. Under herbivore pressure, there was no significant fitness difference between the types, although the fitness of velvety plants was still higher than that of sticky plants. This occurred even though velvety plants sustained more herbivore damage than sticky plants and were more likely to be attacked by most herbivore species present. The fitness difference between the plant types was especially reduced when herbivore-attacked plants were watered, which indicates that sticky plants may have higher tolerance for damage than velvety plants when supplied with a potentially limiting resource. Yet, the maintenance of a fitness deficit (albeit small and nonsignificant) for sticky plants when attacked by herbivores indicates no net benefit associated with the production of glandular trichomes in this first year of our study. These results add to our current understanding that herbivore resistance characters can be costly and raise the question of how this genetic polymorphism is maintained in wild populations.     

No benefit of glandular trichome production in natural populations of Datura wrightii?

Populations of Datura wrightii vary in the frequency of plants that produce glandular trichomes, a resistance trait under the control of a single gene. Such variation may be maintained if the production of glandular trichomes is costly in the absence of herbivory, and if selection imposed by herbivore communities varies spatially or temporally. Here, we document costs in the presence of herbivory for established glandular plants relative to established non-glandular plants growing in natural populations from coastal mountain, Riversidian sage scrub, and Mojave desert habitats in southern California. Damage caused by the herbivore community varied spatially, with significant differences in herbivore-specific damage between plants of the two trichome types and among populations within habitats, although not generally among habitats. Plants with greater canopy size and canopy persistence had higher viable seed production than smaller or more damaged plants, but this relationship was statistically significant only for non-glandular plants. However, the relationship between viable seed production and canopy persistence became significant for glandular plants when damage caused by sap suckers, which do not remove leaf area, was pooled with undamaged leaf area. The high cost exhibited by glandular plants leads us to predict that in the absence of any additional, unknown benefits of producing glandular trichomes, the frequency of these plants should decline in all natural populations of D. wrightii. Received: 13 July 1999 / Accepted: 28 September 1999     

Direct and ecological costs of resistance and tolerance in the stinging nettle

Plant resistance and tolerance to herbivores, parasites, pathogens, and abiotic factors may involve two types of costs. First, resistance and tolerance may be costly in terms of plant fitness. Second, resistance and tolerance to multiple enemies may involve ecological trade-offs. Our study species, the stinging nettle ( Urtica dioica L.) has significant variation among seed families in resistance and tolerance as well as costs of resistance and tolerance to the holoparasitic plant Cuscuta europaea L. Here we report on variation among seed families (i.e. genetic) in tolerance to nutrient limitation and in resistance to both mammalian herbivores (i.e. number of stinging trichomes) and an invertebrate herbivore (i.e. inverse of the performance of a generalist snail, Arianta arbustorum). Our results indicate direct fitness costs of snail resistance in terms of host reproduction whereas we did not detect fitness costs of mammalian resistance or tolerance to nutrient limitation. We further tested for ecological trade-offs among tolerance or resistance to the parasitic plant, herbivore resistance, and tolerance to nutrient limitation in the stinging nettle. Tolerance of nettles to nutrient limitation and resistance to mammalian herbivores tended to correlate negatively. However, there were no significant correlations among resistance and tolerance to the different natural enemies (i.e. parasitic plants, snails, and mammals). The results of this greenhouse study thus suggest that resistance and tolerance of nettles to diverse enemies are free to evolve independently of each other but not completely without direct costs in terms of plant fitness.     

Glandular trichomes of Solanum berthaultii and its hybrids with potato deter oviposition and impair growth of potato tuber moth

Glandular trichomes on foliage of the wild potato species, Solanum berthaultii Hawkes, deter oviposition by the potato tuber moth (PTM), Phthorimaea operculella Zeller and negatively affect other important performance parameters. Oviposition deterring factors are localized in the glandular trichomes of S. berthaultii. When mechanically transferred to foliage of a susceptible potato cultivar, trichome contents reduced egg laying by 97%. Removal of glandular trichomes from S. berthaultii foliage using a combination of chemical and mechanical procedures increased oviposition rates ca. 210-fold. Removal of trichomes also led to increased mobility of larvae on the leaf surface, more leaf feeding, shorter larval development and larger pupae. The resistance conferred by glandular trichomes of S. berthaultii provides an important genetic trait potentially useful for management of PTM.     

Heritable variation in the foliar secondary metabolite sideroxylonal in <Emphasis Type="Italic">Eucalyptus</Emphasis> confers cross-resistance to herbivores

Plants encounter a broad range of natural enemies and defend themselves in diverse ways. The cost of defense can be reduced if a plant secondary metabolite confers resistance to multiple herbivores. However, there are few examples of positively correlated defenses in plants against herbivores of different types. We present evidence that a genetically variable chemical trait that acts as a strong antifeedant to mammalian herbivores of Eucalyptus also deters insect herbivores, suggesting a possible mechanism for cross-resistance. We provide field confirmation that sideroxylonal, an important antifeedant for mammalian herbivores, also determines patterns of damage by Christmas beetles, a specialist insect herbivore of Eucalyptus. In a genetic progeny trial of Eucalyptus tricarpa, we found significant heritabilities of sideroxylonal concentration (0.60), overall insect damage (0.34), and growth traits (0.30–0.53). Population of origin also had a strong effect on each trait. Negative phenotypic correlations were observed between sideroxylonal and damage, and between damage and growth. No relationship was observed between sideroxylonal concentration and any growth trait. Our results suggest that potential for evolution by natural selection of sideroxylonal concentrations is not strongly constrained by growth costs and that both growth and defense traits can be successfully incorporated into breeding programs for plantation trees.     

Noncrop flowering plants restore top‐down herbivore control in agricultural fields

Herbivore populations are regulated by bottom‐up control through food availability and quality and by top‐down control through natural enemies. Intensive agricultural monocultures provide abundant food to specialized herbivores and at the same time negatively impact natural enemies because monocultures are depauperate in carbohydrate food sources required by many natural enemies. As a consequence, herbivores are released from both types of control. Diversifying intensive cropping systems with flowering plants that provide nutritional resources to natural enemies may enhance top‐down control and contribute to natural herbivore regulation. We analyzed how noncrop flowering plants planted as “companion plants” inside cabbage (Brassica oleracea) fields and as margins along the fields affect the plant–herbivore–parasitoid–predator food web. We combined molecular analyses quantifying parasitism of herbivore eggs and larvae with molecular predator gut content analysis and a comprehensive predator community assessment. Planting cornflowers (Centaurea cynanus), which have been shown to attract and selectively benefit Microplitis mediator, a larval parasitoid of the cabbage moth Mamestra brassicae, between the cabbage heads shifted the balance between trophic levels. Companion plants significantly increased parasitism of herbivores by larval parasitoids and predation on herbivore eggs. They furthermore significantly affected predator species richness. These effects were present despite the different treatments being close relative to the parasitoids’ mobility. These findings demonstrate that habitat manipulation can restore top‐down herbivore control in intensive crops if the right resources are added. This is important because increased natural control reduces the need for pesticide input in intensive agricultural settings, with cascading positive effects on general biodiversity and the environment. Companion plants thus increase biodiversity both directly, by introducing new habitats and resources for other species, and indirectly by reducing mortality of nontarget species due to pesticides.     

Searching behaviour of an omnivorous predator for novel and native host plants of its herbivores: a study on arthropod colonization of eucalyptus in Brazil

Adaptation to novel host plants is a much‐studied process in arthropod herbivores, but not in their predators. This is surprising, considering the attention that has been given to the role of predators in host range expansion in herbivores; the enemy‐free space hypothesis suggests that plants may be included in the host range of herbivores because of lower predation and parasitism rates on the novel host plants. This effect can only be important if natural enemies do not follow their prey to the novel host plant, at least not immediately, thus allowing the herbivores to adapt to the novel host plant. Hence, depending on the speed with which natural enemies follow their prey to a new host plant, enemy‐free space on novel host plants may only exist for a limited period. This situation may presently be occurring in a system consisting of the herbivorous moth Thyrinteina arnobia Stoll (Lepidoptera: Geometridae) that attacks various species of Myrtaceae, such as guava (Psidium guajava L.) and jaboticaba (Myrciaria spp.), in Brazil. Since the introduction of eucalyptus (Myrtaceae) species into this country some 100 years ago, the moth has included this plant species in its host range and frequently causes outbreaks, a phenomenon that does not occur on the native host plant species. This suggests that the natural enemies that attack the herbivore on native species are not very effective on the novel host. We tested this hypothesis by studying the searching behaviour of one of the natural enemies, the omnivorous predatory bug Podisus nigrispinus (Dallas) (Heteroptera: Pentatomidae). When offered a choice between plants of the two species, the predators (originally collected in eucalyptus plantations) preferred guava to eucalyptus when both plant species were clean, infested with herbivores, or damaged by herbivores but with herbivores removed prior to the experiments. The bugs preferred herbivore‐damaged to clean guava, and showed a slight preference for damaged to clean eucalyptus. These results may explain the lack of impact of predatory arthropods on herbivore populations on eucalyptus and suggests that eucalyptus may offer an enemy‐free space for herbivores.     

Plant resistance attenuates the consumptive and non‐consumptive impacts of predators on prey

Plant resistance and predation have strong independent and interacting effects on herbivore survival, behavior, and patterns of herbivory. Historically, research has emphasized variation in the consumption of herbivores by enemies. Recent work, however, demonstrates that predators also elicit important changes in the traits of their prey, but we do not know how this is influenced by plant quality. In this study, we quantify how the consumptive and non‐consumptive effects of predators vary along a gradient of plant resistance using tomato plants (Solanum lycopersicum), tobacco hornworms (Manduca sexta), and predaceous stinkbugs (Podisus maculiventris). We manipulated resource quality using three tomato lines that vary in the expression of the jasmonate pathway, a phytohormonal pathway that is central in mediating resistance to insects. Resistant plants had higher levels of defensive proteins and glandular trichomes than low resistance plants. The consumptive and non‐consumptive effects of predators were quantified on the three tomato lines by comparing the impact of ‘lethal’ predators that both kill and scare prey with ‘risk’ predators whose mouthparts were surgically impaired to prevent killing. Across several field experiments, the total cascading effect of predators on plant damage was 80.4% lower on jasmonate‐overex‐pressing (highly resistant) plants compared to that on wild‐type or jasmonate‐insensitive (low resistance) plants. This dramatic attenuation of predator effects was due to a 66% reduction in consumption on high resistance plants, and also because of a 65% decline in non‐consumptive effects. Numerous studies in natural and agricultural habitats have documented that predator effects tend to be weaker on well‐defended plants; our results provide novel mechanistic insight into this pattern by demonstrating that plant resistance substantially weakens both the consumptive and non‐consumptive impacts of predators.     

Herbivory-associated degradation of tomato trichomes and its impact on biological control of Aculops lycopersici

Tomato plants have their leaves, petioles and stems covered with glandular trichomes that protect the plant against two-spotted spider mites and many other herbivorous arthropods, but also hinder searching by phytoseiid mites and other natural enemies of these herbivores. This trichome cover creates competitor-free and enemy-free space for the tomato russet mite (TRM) Aculops lycopersici (Acari: Eriophyidae), being so minute that it can seek refuge and feed inbetween the glandular trichomes on tomato cultivars currently used in practice. Indeed, several species of predatory mites tested for biological control of TRM have been reported to feed and reproduce when offered TRM as prey in laboratory experiments, yet in practice these predator species appeared to be unable to prevent TRM outbreaks. Using the phytoseiid mite, Amblydromalus limonicus, we found exactly the same, but also obtained evidence for successful establishment of a population of this predatory mite on whole plants that had been previously infested with TRM. This successful establishment may be explained by our observation that the defensive barrier of glandular plant trichomes is literally dropped some time after TRM infestation of the tomato plants: the glandular trichome heads first rapidly develop a brownish discoloration after which they dry out and fall over onto the plant surface. Wherever TRM triggered this response, predatory mites were able to successfully establish a population. Nevertheless, biological control was still unsuccessful because trichome deterioration in TRM-infested areas takes a couple of days to take effect and because it is not a systemic response in the plant, thereby enabling TRM to seek temporary refuge from predation in pest-free trichome-dense areas which continue to be formed while the plant grows. We formulate a hypothesis unifying these observations into one framework with an explicit set of assumptions and predictions to be tested in future experiments.     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Identifying Three Ecological Chemotypes of Xanthium strumarium Glandular Trichomes Using a Combined NMR and LC-MS Method

Xanthanolides, as the sesquiterpene lactones, are reportedly the major components for the pharmacological properties of X. strumarium L. species. Phytochemical studies indicated that the glandular structures on the surface of plant tissues would form the primary sites for the accumulation of this class of the compounds. As the interface between plants and their natural enemies, glandular trichomes may vary with respect to which of their chemicals are sequestered against different herbivores in different ecologies. However, to date, no data are available on the chemical characterisation of X. strumarium glandular cells. In this study, the trichome secretions of the X. strumarium species originating from nineteen unique areas across eleven provinces in China, were analysed by HPLC, LC-ESI-MS and NMR. For the first time three distinct chemotypes of X. strumarium glandular trichomes were discovered along with the qualitative and quantitative evaluations of their presence of xanthanolides; these were designated glandular cell Types I, II, and III, respectively. The main xanthanolides in Type I cells were 8-epi-xanthatin and xanthumin while no xanthatin was detected. Xanthatin, 8-epi-xanthatin, and xanthumin dominated in Type II cells with comparable levels of each being present. For Type III cells, significantly higher concentrations of 8-epi-xanthatin or xanthinosin (relative to xanthatin) were detected with xanthinosin only being observed in this type. Further research will focus on understanding the ecological and molecular mechanism causing these chemotype differences in X. strumarium glandular structures.     

Resistance Mechanisms Against Arthropod Herbivores in Cotton and Their Interactions with Natural Enemies

Cotton plants (genus Gossypium) are grown on more than 30 million hectares worldwide and are a major source of fiber. The plants possess a wide range of direct and indirect resistance mechanisms against herbivorous arthropods. Direct resistance mechanisms include morphological traits such as trichomes and a range of secondary metabolites. The best known insecticidal compounds are the terpenoid gossypol and its precursors and related compounds. Indirect resistance mechanisms include herbivore-induced volatiles and extrafloral nectaries that allow plants to attract and sustain natural enemy populations. We discuss these resistance traits of cotton, their induction by herbivores, and their impact on herbivores and natural enemies. In addition, we discuss the use of genetically engineered cotton plants to control pest Lepidoptera and the influence of environmental factors on the resistance traits.     

Trichome morphology provides phylogenetically informative characters for Tremandra, Platytheca and Tetratheca (former Tremandraceae)

Summary Trichomes of Tremandra R.Br. ex DC., Platytheca Steetz and Tetratheca Sm. (Elaeocarpaceae, former Tremandraceae), together with two outgroup species of Elaeocarpus L., are illustrated using scanning electron microscopy, and their distribution on various plant organs is documented. Various trichomes types were identified that relate taxa: simple hairs, stellate hairs, short glandular trichomes, long glandular trichomes, and three forms of tubercules. Both outgroup and ingroup taxa have simple hairs. Stellate hairs are confirmed as unique to Tremandra. Prominent and sculptured multicelled tubercules, some bearing a stout hair, are characteristic of Platytheca. Smaller multicelled tubercules occur in both Platytheca and Tetratheca, except for the Western Australian taxon Te. filiformis Benth. (possibly plesiomorphic). Unicellular tubercules (papilla) characterise two species of Tetratheca. Short glandular trichomes, usually found on the ovary, also occur in both of these genera but not in all species (possibly secondary losses), while long glandular trichomes, usually on stems and leaves, occur only in some groups of Tetratheca. Within Tetratheca, Western Australian taxa that have five-merous flowers fall into three ‘groups’: seven species (together with one from South Australia) that have short glandular trichomes but no long glandular trichomes; six species that have long glandular trichomes but no short glandular trichomes; and four species that have both trichome types. All other species of Tetratheca have four-merous flowers and form two ‘groups’: 12 eastern species (including one from South Australia) that have both short glandular trichomes and long glandular trichomes; 4 western species and six eastern species that lack short glandular trichomes. On the basis of these characters, a phylogenetic hypothesis for the three genera is presented.     

Nutritional suitability and ecological relevance of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> and <Emphasis Type="Italic">Brassica oleracea</Emphasis> as foodplants for the cabbage butterfly, <Emphasis Type="Italic">Pieris rapae</Emphasis>

The thale cress, Arabidopsis thaliana, is considered to be an important model species in studying a suite of evolutionary processes. However, the species has been criticized on the basis of its comparatively small size at maturity (and consequent limitations in the amount of available biomass for herbivores) and on the duration and timing of its life cycle in nature. In the laboratory, we studied interactions between A. thaliana and the cabbage butterfly, Pieris rapae, in order to determine if plants are able to support the complete development of the herbivore. Plants were grown in pots from seedlings in densities of one, two, or four per pot. In each treatment, one, two, or five newly hatched larvae of P. rapae were placed on fully developed rosettes of A. thaliana. In a separate experiment, the same densities of P. rapae larvae were reared from hatching on single mature cabbage (Brassica oleracea) plants. Pupal fresh mass and survival of P. rapae declined with larval density when reared on A. thaliana but not on B. oleracea. However, irrespective of larval density and plant number, some P. rapae were always able to complete development on A. thaliana plants. A comparison of the dry mass of plants in different treatments with controls (= no larvae) revealed that A. thaliana partially compensated for plant damage when larval densities of P. rapae were low. By contrast, single cress plants with 5 larvae generally suffered extensive damage, whereas damage to B. oleracea plants was negligible. Rosettes of plants that were monitored in spring, when A. thaliana naturally grows, were not attacked by any insect herbivores, but there was often extensive damage from pulmonates (slugs and snails). Heavily damaged plants flowered less successfully than lightly damaged plants. Small numbers of generalist plant-parasitic nematodes were also recovered in roots and root soil. By contrast, plants monitored in a sewn summer plot were heavily attacked by insect herbivores, primarily flea beetles (Phyllotreta spp.). These results reveal that, in natural populations of A. thaliana, there is a strong phenological mismatch between the plant and most of its potential specialist insect herbivores (and their natural enemies). However, as the plant is clearly susceptible to attack from non-insect generalist invertebrate herbivores early in the season, these may be much more suitable for studies on direct defense strategies in A. thaliana.     

Preference for outbred host plants and positive effects of inbreeding on egg survival in a specialist herbivore

Inbreeding can profoundly affect the interactions of plants with herbivores as well as with the natural enemies of the herbivores. We studied how plant inbreeding affects herbivore oviposition preference, and whether inbreeding of both plants and herbivores alters the probability of predation or parasitism of herbivore eggs. In a laboratory preference test with the specialist herbivore moth Abrostola asclepiadis and inbred and outbred Vincetoxicum hirundinaria plants, we discovered that herbivores preferred to oviposit on outbred plants. A field experiment with inbred and outbred plants that bore inbred or outbred herbivore eggs revealed that the eggs of the outbred herbivores were more likely to be lost by predation, parasitism or plant hypersensitive responses than inbred eggs. This difference did not lead to differences in the realized fecundity as the number of hatched larvae did not differ between inbred and outbred herbivores. Thus, the strength of inbreeding depression in herbivores decreases when their natural enemies are involved. Plant inbreeding did not alter the attraction of natural enemies of the eggs. We conclude that inbreeding can significantly alter the interactions of plants and herbivores at different life-history stages, and that some of these alterations are mediated by the natural enemies of the herbivores.     

Associational resistance or susceptibility: the indirect interaction between chemically‐defended and non‐defended herbivore prey via a shared predator

Many organisms possess chemical defences against their natural enemies, which render them unpalatable or toxic when attacked or consumed. These chemically‐defended organisms commonly occur in communities with non‐ or less‐defended prey, leading to indirect interactions between prey species, mediated by natural enemies. Although the importance of enemy‐mediated indirect interactions have been well documented (e.g. apparent competition), how the presence of prey chemical defences may affect predation of non‐defended prey in terrestrial communities remains unclear. Here, an experimental approach was used to study the predator‐mediated indirect interaction between a chemically‐defended and non‐defended pest aphid species. Using laboratory‐based mesocosms, aphid community composition was manipulated to include chemically‐defended (CD) aphids Brevicoryne brassicae, non‐defended (ND) aphids Myzus persicae or a mixed assemblage of both species, on Brassica oleracea cabbage plants, in the presence or absence of a shared predator (Chrysoperla carnea larvae). Aphid population growth rates, aphid distributions on host plants and predator growth rates were measured. In single‐species treatments, C. carnea reduced M. persicae population growth rate, but had no significant impact on B. brassicae population growth rate, suggesting B. brassicae chemical defences are effective against C. carnea. Chrysoperla carnea had no significant impact on either aphid species population growth rate in mixed‐species treatments. Myzus persicae (ND) therefore experienced reduced predation in the presence of B. brassicae (CD) through a predator‐mediated indirect effect. Moreover, predator growth rates were significantly higher in the M. persicae‐only treatments than in either the B. brassicae‐only or mixed‐species treatments, suggesting predation was impaired in the presence of B. brassicae (CD). A trait‐mediated indirect interaction is proposed, consistent with associational resistance, in which the predator, upon incidental consumption of chemically‐defended aphids is deterred from feeding, releasing non‐defended aphids from predatory control.     

Costs of glandular trichomes in Datura wrightii: a three-year study

Models accounting for genetic variation for resistance to herbivores within plant populations often postulate a balance between the costs of that resistance and its benefits. The production of glandular trichomes by Datura wrightii was shown to be costly in a previous one-year study because plants producing glandular trichomes (sticky plants), a factor conferring resistance to some insect herbivores, also produced 45% fewer seeds than plants producing nonglandular trichomes (velvety plants) when grown in a common garden. Because sticky plants tended to be larger than velvety plants but produced fewer seed capsules, we postulated an allocation trade-off in which velvety plants are more reproduction-dominated whereas sticky plants are more growth-dominated. If a greater commitment to vegetative growth eventually allows sticky plants to compensate for reduced seed production, we would expect a reduction or elimination of the cost of resistance over time in this perennial plant. We monitored growth, survival, and seed production of plants from defined crosses of local populations for three years in a common garden when exposed to and protected from herbivores, and with and without supplemental water. The majority of plants exposed to herbivores had died by the end of the study. We used standard life-table methods to determine the net reproductive rate (R0) and the finite rate of increase (lambda) of plants of each trichome type. After three years, when plants were protected from herbivores, sticky plants were 187-245% larger than velvety plants, depending upon irrigation treatment, but sticky plants continued to be less efficient in producing seeds per unit of canopy volume. Even though the total seed production of sticky plants eventually equaled that of velvety plants, the advantage of earlier reproduction by velvety plants increased lambda by 55-230% over that of sticky plants, depending upon herbivore and irrigation treatment. Exposure to herbivores reduced lambda by 69-83%, depending upon plant type and irrigation treatment, whereas supplemental irrigation increased lambda by 29-175%, depending upon plant type and exposure to herbivores. Although there was a large allocation trade-off between growth and reproduction, the benefits of such a trade-off did not emerge before most plants were killed by herbivores. The cost of producing glandular trichomes strictly for herbivore resistance continued to exceed its benefits, and in the absence of other, unmeasured benefits from the suite of life-history characters associated with glandular trichome production, natural selection is expected to eliminate this costly resistance trait from D. wrightii populations.     

Parasitoids follow herbivorous insects to a novel host plant,generalist predators less so

The ‘enemy‐free space’ hypothesis predicts that herbivorous insects can escape their natural enemies by switching to a novel host plant, with consequences for the evolution of host plant specialisation. However, if natural enemies follow herbivores to their novel host plants, enemy‐free space may only be temporary. We tested this by studying the colonisation of the introduced tree Eucalyptus grandis (Hill) Maiden (Myrtaceae) by insects in Brazil, where various species of herbivores have added eucalyptus to their host plant range, which consists of native myrtaceous species such as guava. Some herbivores, for example, Thyrinteina leucoceraea Ringe (Lepidoptera: Geometridae), cause outbreaks in eucalyptus plantations but not on guava, possibly because eucalyptus offers enemy‐free space. We sampled herbivores (mainly Lepidoptera species) and natural enemies on eucalyptus and guava and assessed parasitism of Lepidoptera larvae on both host plant species during ca. 2 years. Overall, predators were encountered more frequently on guava than on eucalyptus. In contrast, parasitoids were encountered equally and parasitism rates of Lepidoptera larvae were similar on both host plants. This indicates that herbivores may escape some enemies by moving to a novel host plant. However, this escape may be temporary and may vary with time. We argue that studying temporal and spatial patterns of enemy‐free space and the response of natural enemies to host use changes of their herbivorous prey is essential for understanding the role of natural enemies in the evolution of host plant use by herbivorous arthropods.